The 3rd-generation (3G) Wideband Code-Division Multiple Access (W-CDMA) wireless networks specified by the 3rd-Generation Partnership Project (3GPP) include support for multiple-input multiple-output (MIMO) transmission techniques. (For details, see “3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Physical layer procedures (FDD) (Release 8),” 3GPP TS 25.214, available at http://www.3gpp.org/ftp/Specs/html-info/25214.htm.) In systems built according to these standards, a 2×2 MIMO scheme may be used to transmit the High-Speed Downlink Shared Channel (HS-DSCH) over two transmit antennas via two distinct spatially multiplexed data streams. The two streams use the same channelization codes, but are separated from each other by orthogonal precoding weights.
In these systems, as well as in other systems where high-order modulation schemes such as 16QAM and 64QAM are used, a decision boundary needs to be computed for demodulating symbols from the received signal. The decision boundary is an estimate of the expected amplitude of the complex valued received data symbols, and is used to map (demodulate) received combined symbols to soft values or bits. Thus, for example, the decision boundary is used to map a combined symbol against the constellation map for 16QAM, to obtain 4 soft bit values corresponding to the combined symbol. In a 64QAM scheme, the decision boundary is used to map each received combined symbol to 6 soft bit values.
For MIMO systems, two or more parallel data streams can each use higher order modulation, in which case a separate decision boundary needs to be computed for each data stream. If a decision boundary is incorrect, demodulation and throughput performance of the corresponding data stream will suffer. Because the correct value of the decision boundary changes as the radio channel is fading, an estimate of the decision boundary needs to be recomputed from time to time, especially if the wireless receiver is moving. In conventional systems, a decision boundary for a given data stream is often computed by simply calculating an average over absolute values of several combined symbols of the data stream. In many receivers, an estimate of the decision boundary is computed once per transmission time slot, although other intervals are possible.
In a MIMO system, because of imperfections in the radio propagation channel between the transmitting base station and a mobile terminal, the two streams will interfere with each other. This interference is referred to as code reuse interference. For optimal performance, a MIMO receiver needs to suppress or cancel this interference. In addition to suppressing code reuse interference, a MIMO receiver also needs an estimate of the code reuse interference power to compute accurate channel quality reports for feeding back to the base station. If the receiver computes channel estimates based on pilot channel symbols (e.g., the W-CDMA Common Pilot Channel, or CPICH), the ratio of the traffic channel power (e.g., the W-CDMA High-Speed Physical Downlink Shared Channel, or HS-PDSCH) to the pilot channel power, per channelization code must be known or estimated. This per-code traffic-channel-to-pilot power ratio αPC is used when suppressing or cancelling the code reuse term and may also be used to calculate an estimate of the received signal-to-interference-plus-noise ratio (SINR) for channel quality reporting.
One approach to suppressing code reuse interference in a Generalized Rake (G-Rake) receiver is described in U.S. Patent Application Publication No. 2008/0152053, titled “Method and Apparatus for Determining Combining Weights for MIMO Receivers” and published 26 Jun. 2008, the entire contents of which are incorporated by reference herein. With this approach, a receiver uses scaling parameters representing the normalized per-code energy allocated to each transmitted stream to calculate combining weights that suppress the cross-stream interference. These same scaling parameters may also be used to calculate the estimated code reuse interference power for the purposes of preparing channel quality reports.
Several techniques for estimating the per-code traffic-channel-to-pilot power ratio αPC in a MIMO system are disclosed in several patent application publications, including U.S. Patent Application Publication No. 2009/0213910 and U.S. Patent Application Publication No. 2009/0213909, each of which corresponds to a patent application filed Feb. 25, 2008 and both of which are titled “Code Power Estimation for MIMO Signals.” The entire contents of both of these publications are incorporated by reference herein. Notwithstanding the disclosures in these publications, improved techniques for estimating decision boundaries for use in detecting symbol values from a received signal are still needed.